FirebaseDatabase Framework Reference

FIRMutableData

@interfaceFIRMutableData:NSObject

A FIRMutableData instance is populated with data from a Firebase Database location.
When you are using runTransactionBlock:, you will be given an instance containing the current
data at that location. Your block will be responsible for updating that instance to the data
you wish to save at that location, and then returning using [FIRTransactionResult successWithValue:].

To modify the data, set its value property to any of the native types support by Firebase Database:

NSNumber (includes BOOL)

NSDictionary

NSArray

NSString

nil / NSNull to remove the data

Note that changes made to a child FIRMutableData instance will be visible to the parent.